Giant cell arteritis (GCA) is reported world-wide. However, the incidence varies greatly in different geographic regions with the highest incidence rates from Scandinavian countries and North American populations of the same descent. The etiopathogenesis of GCA is incompletely understood. Although data on a positive correlation between the occurrence of infection and the onset of GCA as well as rhythmic fluctuations in disease incidence have been reported, no statistically significant periodicity has so far been found regarding the annual incidence of GCA. Seasonal variations may indicate a role of infection or other environmental factors in the pathogenesis of GCA.